1. Field of the Invention
This invention relates to control systems for controlling the speed of electric motors, and in particular to two-speed motor control systems for controlling pole-amplitude-modulated (PAM) motor systems.
2. Description of the Prior Art
A pole-amplitude-modulated (PAM) motor is a two-speed electric motor having one set of motor windings designed so that the one set of windings can be reconfigured using a switching mechanism so as to achieve any two particular speeds in the PAM motor itself. Such a motor is advantageous in that it eliminates the requirement of a second set of motor windings to achieve a second speed thereby reducing the overall weight and size of the required motor. In addition, the PAM motor is energy efficient.
The components of a PAM motor system include, in one embodiment, a speed switch for effecting the reconfiguration of the motor windings and a motor breaker switch for electrically protecting the speed switch. It is important in this embodiment of a PAM motor system that, when changing the speed of the PAM motor, the motor breaker switch and the speed switch be operated, that is opened and closed, in the proper sequence. This is because, for example, back emf generated by the flux decay of the PAM motor may damage the speed switch if the sequence of operation is improper, that is, for example, if the speed switch is changed under motor load.
A control system is required for controlling the sequence of operation of the components of the PAM motor so as to assure safe and reliable operation. It is especially desirable that a control system be provided to a user along with the PAM motor so as to avoid requiring the user to design a control system which may not properly govern the operation of the PAM motor. For example, a user-designed control system may not include operating paths for all the possible contingent operating conditions to assure reliable and safe operation of the PAM motor either because the user is not aware of certain contingent operating conditions or because the user-designed system concept is faulty in that there are incomplete or improper system contingency operating paths.
One typical prior art control system includes digital logic and a grey code design having electronic timers for sequencing the operation of the components of the PAM motor system. Since a grey code is only valid where one digital bit at a time is changed from one digital word to another, the prior art grey code control system is limited in its flexibility to include a variety contingencies in an operating path. This is because it might be desirable to follow a contingency operating path requiring a change of more than one digital bit. Such a change is invalid using a grey code. In addition, there have been possible race conditions in the prior art scheme which could result in an improper sequence of operation.